The present invention relates to an exhaust system for receiving an exhaust gas, wherein the exhaust system comprising a first SCR catalyst, a second SCR catalyst positioned downstream of said first SCR catalyst, a first injector provided upstream of said first SCR catalyst, and a second injector provided upstream of said second SCR catalyst.
Engines, in particular diesel-powered combustion engines but also other engines known in the art, produce exhaust gases which contain several air pollutants, including carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides NO and NO2 (NOx) as well as particulate matter (PM) containing carbonaceous matter, or soot.
With increased attention to environmental problems and health hazard prevention, exhaust emission regulations are becoming more and more stringent.
To reduce the amount of NOx in the exhaust gases, some engines are equipped with Selective Catalytic. Reduction (SCR) systems that convert a mixture of NOx and ammonia (NH3) into nitrogen gas (N2) and water (H2O). For example, document US 2008/0060348 A1 shows an exhaust system for reducing NOx comprising a first SCR catalyst, a second SCR catalyst, and a particulate filter positioned between said first and second SCR catalyst. This system however exhibits a limited efficiency for removal of the above mentioned pollutants during low-temperature exhaust conditions, such as after cold start of the engine, or during low-temperature operation, e.g. low-speed urban driving conditions. Consequently, in order to combine an efficient NO2-based oxidation of stored PM in the particulate filter with a maximum efficiency for removal of the above mentioned NOx pollutants during low-temperature exhaust conditions, further improvement in reductant dosing definition and a dedicated operating strategy according to the present invention is beneficial.
Another problem with efficient exhaust treatment systems in general is relatively short refill-intervals of the ammonia-source that is required for operation of the SCR catalyst.
Still another problem with efficient exhaust treatment systems is the physical size of the system and the limited space available in modern vehicles, rendering packaging of the exhaust aftertreatment system difficult.
There is thus a need for an improved exhaust aftertreatment system that removes the above mentioned disadvantage.
It is desirable to provide an inventive exhaust system for receiving an exhaust gas and a method for operating the system where the previously mentioned problems are avoided. Said exhaust system comprising a first SCR catalyst, a second SCR catalyst positioned downstream of said first SCR catalyst, a first injector provided upstream of said first SCR catalyst, and a second injector provided upstream of said second SCR catalyst.
According, to an aspect of the present invention, said exhaust system further comprising a gaseous ammonia supply device being fluidly connected to said first injector for supplying gaseous ammonia to said exhaust gas by said first injector, and an ammonia-containing reductant reservoir being fluidly connected to said second injector for supplying a fluid ammonia-containing reductant, such as urea, to said exhaust gas by said second injector. Said first SCR catalyst has a smaller volume than said second SCR catalyst for a fast warm-up of said first SCR catalyst.
The inventive arrangement has preferably two clear operating modes: low-temperature exhaust gas operating mode and high-temperature exhaust gas operating mode. During the low-temperature exhaust gas operating mode, gaseous ammonia is injected upstream of the first SCR catalyst, which efficiently reduces NOx of the exhaust gases. In general, no ammonia-containing reductant is injected by the second injector in this operating mode. During the high-temperature exhaust gas operating mode, ammonia-containing reductant is injected upstream of the second SCR catalyst, which efficiently reduces NOx of the exhaust gases. Generally, no gaseous ammonia is injected by the first injector in this operating mode. The inventive arrangement results not only in significantly improved NOx emissions control during cold start and low-temperature exhaust operating conditions in general, but also in maintained long service intervals of the exhaust aftertreatment system, as well as improved exhaust system packaging.
The inventive exhaust system is designed to perform efficient NOx reaction by means of the first SCR catalyst also at a reduced exhaust temperature level compared with the prior art, such that catalytic NOx reduction can be performed at an earlier stage upon engine cold-start. There are a number of underlying reasons behind this technical effect:
The first SCR catalyst exhibits a smaller volume than the second SCR catalyst. For any given substance, the heat capacity of a body is directly proportional to the amount of substance it contains. As a consequence, the temperature of a smaller volume SCR catalyst may be more rapidly increased than a larger volume SCR catalyst due to the smaller heat capacity of the smaller SCR catalyst compared with the larger SCR catalyst.
The SCR catalyst requires ammonia for the catalytic removal of NOx emissions in the exhaust gases. When injecting a fluid ammonia-containing reductant, such as urea, the decomposition reaction of said reductant to active reagent gaseous ammonia within the exhaust passage requires a certain temperature level of the exhaust gases at the injection position for substantially complete decomposition, e.g. normally around 200° C. Injection of the ammonia-containing reductant can thus not be initiated much below said temperature level, thereby strongly limiting the NOx emission abatement efficiency. However, by supplying gaseous ammonia instead if a fluid ammonia-containing reductant, there is no requirement to wait with injection until the exhaust gases have reached said temperature level. Consequently, the NOx emission abatement can be initiated at a lower temperature, allowing the full active temperature window of the SCR catalyst to be employed, e.g., down to temperatures around 150° C.
Moreover, the inventive arrangement as a consequence also results in improved NOx emissions control upon entering a low-temperature exhaust operating conditions, such as traffic jams, low-speed urban driving conditions, or the like, because the system may always go back from the high temperature exhaust gas operating mode to the low temperature exhaust gas operating mode as described above when necessary.
A further inventive advantage of having a small first SCR catalyst is the simplified arrangement of said catalyst close to the outlet of the engine or turbocharger, where the space is very limited. The relatively small size of the first SCR catalyst allows closer arrangement thereof to the engine. Hence, the limitation in size of the first SCR catalyst is a factor supporting the improved NOx emission reduction of the present invention. The addition of gaseous ammonia upstream the first SCR catalyst enables a very short mixing distance, utilizing the turbulence created directly downstream the turbocharger outlet, further enabling a compact packaging solution.
Another inventive advantage of an aspect of the present invention results from the specific supply arrangement of the active reagent ammonia to the first and second SCR catalyst of the invention. The combination of supplying gaseous ammonia to the first SCR catalyst and fluid ammonia-containing reductant, such as urea, to the second SCR catalyst allows, by means of intelligent dosing strategy of said first and second ammonia source, extended service intervals of the exhaust aftertreatment system, rendering said system more efficient, reliable and cost effective. For example, by supplying, gaseous ammonia via the first injector to the first SCR catalyst only during the low-temperature exhaust gas operating mode, such as cold-start and certain urban driving conditions, and upon reaching a certain exhaust gas temperature at the second SCR catalyst, supplying ammonia-containing reductant via the second injector to the second SCR catalyst while stopping the supply of gases ammonia, the first as second SCR catalyst are operated more or less non-simultaneously. This type of dosing strategy, which is possible by means of the inventive arrangement, does not only enables a very high NOx conversion efficiency without any penalty to fuel economy but also results in significantly extended service intervals of the exhaust aftertreatment system, i.e. longer time periods between refill or exchange of the ammonia sources. It is of course possible to have a less distinct transition between injection at the first or the second injector, and catalytic operation of the first or second SCR catalyst and both SCR catalysts may be configured to be simultaneously catalytically active during a transition period.
An advantage of having, a substantially non-simultaneous dosing strategy is more efficient N02-based PM regeneration process of a particulate filter when such is provided in the exhaust system, because the N02-based PM regeneration process, which is essentially only possible when dosing of gaseous ammonia to the first SCR catalyst has been stopped, results in less thermal degradation of the catalytic components and a lower fuel economy penalty, as described more in detail in the detailed description below. In the temperature range where the N02-based PM regeneration process in active, e.g. 250-450° C., the dosing of ammonia-containing reductant, e.g. urea, to the second SCR catalyst is fully active so that the emissions compliance is maintained even if the dosing of gaseous ammonia to the first SCR catalyst has been stopped.
According to an aspect of the invention, the volume of said first SCR catalyst is within a range of 5% to 60% of the volume of said second SCR catalyst, specifically within a range of 5% to 40% of the volume of said second SCR catalyst, and more specifically within a range of 10% to 25% of the volume of said second SCR catalyst. Typical values of said volumes in a diesel engine for a heavy truck is about 5-10 liters for the first SCR catalyst, and about 40 liters for the second SCR catalyst. The limited volume of the first SCR catalyst results in faster warm-up and simplified positioning thereof close to the engine or turbocharger outlet. Due to the limited volume, the first SCR catalyst may not have the capacity to alone provide sufficient catalytic removal of NOx emission in the upper operating load region of the engine, i.e. upon high power demand, but the capacity is sufficient in the engine operating conditions normally associated with cold-start and low-temperature exhaust gas urban driving, i.e. a relatively low operating engine load.
According to an aspect of the invention, said exhaust system further comprises an ammonia oxidation catalyst located downstream of said second SCR catalyst. An ammonia oxidation catalyst at the end of the exhaust aftertreatment system can remove any residual ammonia in the exhaust gas that did not react in the second SCR catalyst, by means of oxidation of residual ammonia
According to an aspect of the invention, said exhaust system further comprising a particulate filter positioned between said first and second SCR catalyst. Depending on the type of fuel used, more or less soot and particulate matter is produced. Diesel fuel for example produces more soot and requires thus often a particulate filter for fulfilling legal emission requirements, whereas fuel such as natural gas or dimethyl ether produces generally small amounts of soot, thereby often eliminating the need of a particulate filter.
According to an aspect of the invention, said exhaust system further comprising an oxidation catalyst located downstream of said first SCR catalyst and upstream of said particulate filter. The oxidation catalyst serves to oxidise hydrocarbons and carbon monoxide into carbon dioxide and water. The oxidation catalyst also increases the exhaust temperature. The oxidation catalyst is arranged upstream of the second SCR catalyst, which is configured to be used as sole SCR catalyst during normal highway driving and other normal and high-temperature exhaust operating conditions.
According, to an aspect of the invention, the first SCR catalyst is a vanadia-based (e.g. V205/Ti02 W03) catalyst, and said second SCR catalyst is a zeolite-based catalyst. Using a vanadia-based catalyst as first SCR catalyst is advantageous because this type of catalyst does not require NO2 for efficient selective catalytic reduction of NOx emissions. There is essentially no N02 available in the exhaust gas directly after the engine or turbocharger outlet port. Furthermore, in the case of misfueling with high-sulphur fuel (e.g. >300 ppm S), some zeolite-based SCR catalysts require elevated temperatures, e.g. 600° C., for removal of adsorbed sulphur species to regain the SCR catalyst performance. A vanadia-based SCR catalyst is generally very sulphur tolerant and does not require elevated temperatures for removal of adsorbed sulphur species.
The second SCR catalyst is preferably implemented by a zeolite-based catalyst because of its wide active temperature window, good heat-resistance and effective NOx reduction, but the second SCR catalyst may alternatively also be implemented by a vanadia-based catalyst.
According to an aspect of the invention, the gaseous ammonia supply device can be implemented in several ways. For example, one or more gas bottles holding pressurised ammonia gas can be provided and coupled to the first injector, such that gaseous ammonia may be injected upstream of the first SCR catalyst immediately upon a cold-start, or similar conditions. Replacement and handling of the gas bottles is also relatively easy. According to an alternative embodiment, a storage container may be provided that is configured to store an alkaline earth metal chloride salt, which functions as a source of said gaseous ammonia. The container is preferably heated by electrical wires or the like, thereby facilitating release of gaseous ammonia. Transporting ammonia in a solid storage medium, such as alkaline earth metal chloride salt, results in satisfactory safety and handling of the ammonia source, and only a small amount of heat is required to release the gaseous ammonia. According to yet another alternative, the gaseous ammonia supply device may comprise a storage container holding a solution of ammonia dissolved in a solvent, such as water.
According to an aspect of the invention, said first injector is formed by metal pipe that passes through a side wall of an exhaust passage of said exhaust system, and exhibits an discharge opening within said exhaust passage, such that gaseous ammonia from said gaseous ammonia supply device can be supplied to an exhaust gas flow within said exhaust passage by means of said metal pipe. As previously mentioned, the first SCR catalyst is preferably positioned very close to the engine or turbocharger outlet for rapid heat-up. However, the first injector, which is configured to supply the first SCR catalyst with gaseous ammonia and is arranged upstream of the first SCR catalyst, must consequently be positioned even closer to said outlet. There is thus a problem of extremely high heat at the position of the first injector. By forming the first injector merely by metal pipe that is arranged to receive gaseous ammonia from the ammonia supply device and release the gaseous ammonia within the exhaust passage by means if a discharge opening, the first injector is extremely heat resistant, leading to a reliable and cost-effective design and implementation of the first injector.
According, to an aspect of the invention, said first injector is free from parts made of thermo-plastic material, or other heat sensitive materials. As mentioned, providing a heat-resistant first injector improves system reliability and reduces costs. The addition of gaseous ammonia upstream the first SCR catalyst also enables a very short mixing distance compared to the corresponding injection of ammonia-containing fluid, such as urea.
According to an aspect of the invention, an electronic controller is configured to control injection of gaseous ammonia by said first injector, such that supply of gaseous ammonia to said exhaust gas by said first injector is limited to a operation mode where a temperature Tscm associated with said first SCR catalyst is above a first value Ti, and a temperature TSCR2 associated with said second SCR catalyst is below a second value T2, thereby at least partly facilitating high NOx conversion efficiency of said exhaust gas at temperatures when NOx conversion efficiency of said exhaust has by said second SCR catalyst is low. The lower end of the active temperature window of the first SCR catalyst is defined by said first value T-i, below which temperature level efficient NOx reduction is no longer accomplished.
Furthermore, as previously described, the system is configured to stop injection of gaseous ammonia upstream of the first SCR catalyst when the temperature TSC2 associated with said second SCR catalyst has reached a second value T2.
Another inventive advantage of an aspect of the present invention results from the specific supply arrangement of the active reagent ammonia to the first and second SCR catalyst of the invention. The combination of gaseous ammonia supply to the first SCR catalyst and fluid ammonia-containing reductant supply, such as supply of urea, to the second SCR catalyst allows by means of intelligent dosing strategy of said ammonia sources, extended service intervals of the exhaust aftertreatment system, rendering said system more efficient, reliable and cost effective. For example, by supplying gaseous ammonia via the first injector to the first SCR catalyst only during low-temperature exhaust gas operating conditions, such as cold-start and certain urban driving conditions, and upon reaching a certain exhaust gas temperature at the second SCR catalyst, supplying ammonia-containing reductant via the second injector to the second SCR catalyst while stopping the supply of gases ammonia, the first and second SCR catalysts are operated more or less non-simultaneously. This type of dosing strategy, which is possible by means of the inventive arrangement, does not only enable a very high NOx conversion efficiency without any penalty to fuel economy but also result in significantly extended service intervals of the exhaust aftertreatment system, i.e. longer time periods between refill or exchange of the ammonia sources. More specifically, the exchange of ammonia-containing storage containers can be performed during normal vehicle service intervals for e.g. engine oil.
According to an aspect of the invention, the temperature Tscm associated with said first SCR catalyst is the temperature of the exhaust gas in the region directly upstream of said first SCR catalyst, and the temperature TSCR2 associated with said second SCR catalyst is the temperature of the exhaust gas in the region directly upstream of said second SCR catalyst, and said first value T-i is about 120° C., preferably about 150° C., and said second value T2 is about 270° C., preferably about 250° C.
According to an aspect of the invention, an electronic controller is configured to control injection of fluid ammonia-containing reductant by said second injector, such that supply of ammonia-containing reductant to said exhaust gas by said second injector is limited to an operation mode where a temperature level TSCR2 associated with said second SCR catalyst is above a third value T3, which corresponds to a lower end of the active temperature window of the second SCR catalyst. The second value T2 may be set equal to T3, such that injection at the first injector ends when injection at the second injector begins.
Alternatively, the second value may be arranged a certain level above the third value T3, such that a certain injection overlap occurs by the first and second injectors. In other words, the overlapping temperature range, which is defined by the second value T2 and third value T3, signifies that both the first and second SCR catalysts are temporarily in a simultaneous operating mode during a hand over and start-up of NOx catalytic reduction from one SCR catalyst to the other SCR catalyst. The electronic controller allows fast and economical control of the exhaust aftertreatment system in general, especially with respect to the particular dosing strategy used by the inventive system.
According to an aspect of the invention, said temperature TSCR2 associated with said second SCR catalyst is the temperature of the exhaust gas in the region directly upstream of said second SCR catalyst, and said third value T3 is 200° C., preferably 250° C.
According to an aspect of the invention, said first SCR catalyst is arranged less than 0.6 meters downstream from an exhaust manifold outlet or turbo exhaust outlet of said engine, preferably less than 0.4 meters downstream from an exhaust manifold outlet or turbo exhaust outlet, and more preferably less than 0.25 meters downstream from an exhaust manifold outlet or turbo exhaust outlet. By arranging the first SCR catalyst more close to the exhaust manifold outlet, or turbo exhaust outlet if a turbocharger is provided, the exhaust gases passing through the first SCR catalyst will be warmer, thereby facilitating a more rapid warm-up of said catalyst.
According to an aspect of the invention, an electronic controller is configured to control injection of fluid ammonia-containing reductant by said second injector. The electronic controller allows fast and economical control of the exhaust aftertreatment system in general, especially with respect to the particular dosing strategy used by the inventive system.
According to an aspect of the invention, said second injector is provided downstream of said particulate filter. This arrangement prevents ammonia-containing reductant from entering the DPF.
According to another aspect of the invention, a method of operating an exhaust system comprising a first SCR catalyst; a second SCR catalyst positioned downstream of said first SCR catalyst; a first injector provided upstream of said first SCR catalyst; and a second injector provided upstream of said second SCR catalyst, wherein the method comprises the steps of controlling injection of gaseous ammonia by a first injector, such that supply of gaseous ammonia to said exhaust as by said first injector is limited to a operation mode where a temperature (TSCR1) associated with said first SCR catalyst is above a first value (T-i), and a temperature (TSCFS) associated with said second SCR catalyst is below a second value (T2), thereby at least partly facilitating high NOx conversion efficiency of said exhaust gas at temperatures when NOx conversion efficiency of said exhaust gas by said second SCR catalyst is low.
According to a first embodiment of the method, the temperature (TSCR1) associated with said first SCR catalyst (10) is the temperature of the exhaust gas in the region directly upstream of said first SCR catalyst (10), and said temperature (TSCR2) associated with said second SCR catalyst (16) is the temperature of the exhaust gas in the region directly upstream of said second SCR catalyst (16), and said first value (T is 120° C., preferably 150° C., and said second value (T2) is 270° C., preferably 250° C.
According to a second embodiment, the method comprises the step of controlling injection of fluid ammonia-containing reductant by said second injector, such that supply of ammonia-containing reductant to said exhaust gas by said second injector is limited to an operation mode where a temperature level (TSCFS) associated with said second SCR catalyst is above a third value (T3).
According to a first embodiment of the method, said temperature (TSCR2) associated with said second SCR catalyst is the temperature of the exhaust gas in the region directly upstream of said second SCR catalyst, and said third value (T3) is 200° C., preferably 250° C.